Apple devices heating up on setup or after a OS upgrade are nothing new. 

I suspect testing revealed the heat problem ahead of the iPhone 15 Pro launch, but Apple decided it'd be edge case enough to safely ignore for the product launch.

Oopsie.

https://appleinsider.com/articles/23/09/30/apple-confirms-ios-17-fix-for-overheating-iphones-is-on-the-way